This invention relates to the field of electrical keyboards. More particularly, this invention relates to the field of membrane keyboards having opposed or facing circuit layers which are separated from each other and have contact areas or elements which may be selectively brought into contact to complete an electrical circuit.
Membrane keyboards of the general type with which this invention is concerned are well known in the art. These keyboards conventionally have a pair of circuit layers, one fixed and one movable, separated by a spacer or separator layer. The circuit layers are sheets of insulating material, typically Mylar, with circuit patterns thereon. These circuit patterns face each other and are separated by a spacer, also typically Mylar, which has apertures at the location of aligned contact elements on the fixed and movable circuit sheets. Electrical switching is effected by applying finger or other pressure to specific locations on one of the circuit sheets to move a contact on that circuit sheet through an aperture to make contact with a contact element on the other circuit sheet. The fixed and movable circuit layers and the spacer may be separate sheets of material, or any two or three of those sheets may be formed from a single sheet of material folded over in any desired fashion. Keyboards of this configuration are generally formed in a laminate construction with the layers bonded together, sealed or otherwise fixed against relative lateral movement between the layers. The assembly may also include an overlay sheet with indicia of one kind or another to identify key locations and a backer plate to support the assembly.
The apertured spacer layer requires the production, handling and assembly or processing of the separate component for each keyboard assembly, and requires the production of a die or other machinery for each spacer design. The apertured spacer element also has a discrete effect on design and operation of the keyboard. Actuating forces and operation of a membrane type keyboard are affected by both the thickness of the spacer element and by the size of the aperture. Furthermore, in applications where very large key areas are desired (e.g., a three inch diameter circular key pad or a three inch by three inch key pad) the apertured spacer becomes unreliable and unacceptable, because the movable circuit layer may sag through the large aperture and come into contact with the lower circuit layer to produce undesirable short circuiting of switch contacts.